This is the Robinson Racing Xtra Hard 5mm Bore Steel 12-Tooth 1.0 Module Pinion Gear. This works in brushless setups for the Traxxas Revo 2.5, 3.3, and T-Maxx 3.3, and it also works with most 1/8 scale brushless conversions for Associated, Losi, HPI, Ofna, Jammin, and Kyosho.